Temperature control mechanisms for a micro heat pipe catheter

ABSTRACT

A catheter that provides precise temperature control is disclosed herein. The catheter may use a variety of passive heat pipe structures alone or in combination with feedback devices. The catheter is particularly useful for treating diseased tissue that cannot be removed by surgery.

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of patent application Ser.No. 550,519, filed Jul. 10, 1990, now U.S. Pat. No. 5,190,539.

FIELD OF THE INVENTION

The present invention relates generally to medical devices and, moreparticularly, to temperature control mechanisms and uses for a microheat pipe catheter.

DESCRIPTION OF THE RELATED ART

Millions suffer from cancer, and new techniques for cancer treatment arecontinually being developed. The use of local hyperthermia (elevatingthe temperature of a cancerous part of the body to a slightly highertemperature) has received increased attention over the past few years.Heating a cancerous tumor, including the edges of the tumor, totherapeutic temperatures of 42.5° C. (108.5° F.) to 43.0° C. (109.4° F.)for periods of 20 to 30 minutes will in most cases destroy the rapidlygrowing cancer cells and arrest tumor growth.

Total body temperatures above 41.8° C. (107.2° F.) are detrimental tothe functions of the central nervous system, heart, liver, and kidneys,and may even cause histologically obvious damage to tissue cells,whereas tumorcidal effects are generally not observed below 42.5° C.(108.5° F.). At brain temperatures of over 41.8° C. (107.2° F.), themechanism that regulates body temperature can become incapacitated, andthere is danger of `malignant` or `runaway` hyperthermia. Further,temperatures of up to 45° C. (113.0° F.) may cause soft tissue necrosesand fistulas as well as skin burns. Therefore, accurate temperaturecontrol of a localized area is critical to successful hyperthermia.

There is a significant need for development of a simple hyperthermiadevice which will generate a precisely controllable temperature. Theheat should be confined to the diseased region to minimize the risk ofdamage to the surrounding normal tissue and to preserve normal bodilyfunctions. Local hyperthermia should elevate the temperature of acancerous tumor to a therapeutic level while maintaining the temperatureof the surrounding tissue at, or near normal levels.

Numerous heating methods for tumor treatment have been proposed over thepast few decades, and several methods are currently being practiced.These heating techniques may be classified from a clinical point of viewas non-invasive and invasive.

Non-invasive hyperthermia techniques focus electromagnetic or ultrasonicenergy on the region to be heated. This energy heats the body tissues tothe desired temperatures. However, it is not possible to confine thisenergy to the diseased tissue, and the resulting effect is regionalheating rather than local heating. Due to this regional heating, thistechnique often exhibits large temperature fluctuations due tovariations in blood flow and thermal conductivity of the tissue. Tobetter focus the energy to minimize regional heating, the wavelength ofthe energy beam must be small compared to the tumor's dimensions. Anundesirable side effect of reducing the wavelength renders the techniqueuseful for treating diseased areas only a few centimeters into the body.Another limitation is caused by bones being very strong absorbers ofultrasonic waves and air cavities being almost perfect reflectors. Bonesmay absorb a disproportionate amount of energy, and the reflectionscause energy to disperse uncontrollably.

Invasive heating techniques, as compared with non-invasive techniques,are typically better for achieving therapeutic temperature levelswithout appreciable heating of normal tissues, regardless of the tumorgeometry. Invasive heating techniques include the perfusion of theextremities with extracorporally heated blood and the irrigation of theurinary bladder with heated saline. Other invasive heating techniquesinclude placing heating elements directly into the tumor. The use of anumber of heating elements facilitates the regulation of temperaturethroughout the tumor.

Invasive hyperthermia devices include: (1) sets of implanted electrodesconnected to a radio frequency generator; (2) combinations of implantedand external electrodes; (3) implanted microwave antennas; and (4)implanted or injected thermoseeds. Each of these invasive devicesexhibit drawbacks.

The use of implanted electrodes, while simple, involves placing an arrayof needles into the tumor and connecting them to an RF generator. Thetemperature field for such electrodes is very difficult to control, andthe volume that can be heated effectively is rather small, requiringmany implants. Therefore, this technique is complicated, and thearrangement may result in non-uniform heating.

Electrodes require connections to a power source, and many electrodesare required to treat most tumors. The large number of connection wiresor coaxial feed lines associated with the electrodes are cumbersome andmay overheat.

Implanting microwave antennas or thermoseeds are probably the mostpopular invasive heating techniques. Generally, an array of antennas orthermoseeds is implanted in the tumor and left in place for the durationof the treatment. The antennas absorb externally-applied microwaveenergy, and the thermoseeds absorb externally-applied magnetic energy.Each antenna and thermoseed acts as a small heating unit, transferringheat to the tumor by conduction. The antennas and thermoseeds requirecareful placement in the tumor to optimize local heating of the tumor.This is particularly so with thermoseeds, because their orientation withrespect to the induced magnetic energy determines their heating pattern.Furthermore, because all antennas and thermoseeds are heated to the sametemperature by externally-applied energy, areas with poor blood flow mayoverheat while areas with high blood flow may not attain therapeutictemperatures.

The present invention is directed to overcoming or minimizing one ormore of the problems discussed above.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a micro heat pipe catheter which comprises a shaft having afirst end, a second end, and an intermediate portion extendingtherebetween. The first end is needle-like in shape for penetrating softtissue, and the second end is adapted to couple to a thermal transferelement. The shaft contains a channel that extends between the two endsof the shaft and terminates into a fluid reservoir at the first end. Thechannel is chargeable with a working fluid. A thermally insulativebarrier is provided along the intermediate portion of the shaft toprotect surrounding tissue from damaging temperature change. The shaftalso contains a temperature sensing thermocouple.

In accordance with a further aspect of the present invention, there isprovided a micro heat pipe catheter that includes a shaft having a firstend and a second end. The first end is needle-like in shape. A channelis disposed within the shaft and is chargeable with a quantity of aworking vapor. An insulating layer is disposed along the shaft betweenthe first end and the second end. A valve is disposed within the channelfor controlling vapor flow within the channel.

In accordance with another aspect of the present invention, there isprovided a method of treating diseased tissue. The method includes thesteps of (1) inserting a needle-like end of a micro heat pipe catheterinto the diseased tissue; (2) maintaining the needle-like end of themicro heat pipe catheter within a prescribed temperature range; and (3)thermally insulating a portion of the micro heat pipe catheter toprotect healthy tissue from thermal damage.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings in which:

FIG. 1 is a side view of a catheter;

FIG. 2 is a cross-sectional side view of a micro heat pipe catheter;

FIGS. 2A, 2B, 2C, and 2C-I are cross-sectional views taken along lines2A--2A, 2B--2B, and 2C--2C, respectively in FIG. 2.

FIG. 3 is a schematic view of a passively-controlled, gas-loaded heatpipe;

FIG. 4 is a schematic view of an actively-controlled, gas-loaded heatpipe;

FIG. 5 is a perspective view of a micro heat pipe catheter showing thelocation of the temperature sensing thermocouples;

FIGS. 5A and 5B are cross-sectional views taken along lines 5A--5A andB--5B, respectively, in FIG. 5;

FIG. 6 is a schematic view of a vapor-modulated variable conductanceheat pipe;

FIG. 7 is a schematic view of another vapor-modulated variableconductance heat pipe;

FIG. 8 is a schematic view of a liquid-modulated heat pipe;

FIG. 9 illustrates a method of treating diseased tissue located near thespinal cord;

FIG. 10 illustrates a method of treating diseased tissue located in theprostrate gland;

FIG. 11 illustrates a method of treating diseased tissue located in thebreast;

FIG. 12 illustrates a method of treating diseased tissue located nearthe eye; and

FIG. 13 illustrates a method of treating diseased tissue located in thebrain.

The present invention is susceptible to various modifications andalternative forms. Specific embodiments have been shown by way ofexample in the drawings and will be described in detail herein. However,it should be understood that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents, and alternatives following within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hyperthermia or hypothermia is generally used in addition to surgery,radiation, and chemotherapy, rather than alone as the first line oftreatment. Hyperthermia or hypothermia, when used in new or previouslytreated tumors, is found to produce strong tumorcidal effects. As willbecome apparent, the catheter disclosed herein replaces some existinghyperthermia or hypothermia treatment techniques for some canceroustumors and increases the number of individuals who may be treated,because it may be used to treat deep tumors that cannot be effectivelytreated with other techniques.

Turning now to the drawings and referring initially to FIG. 1, a microheat pipe catheter 8 is illustrated. The catheter 8 is preferablyconstructed of stainless steel or other biocompatible material in amanner similar to the construction of hypodermic needles. The catheter 8has a shaft 10, a needle-like end 12 and a heating or cooling source end14. The needle-like end 12 is adapted to be inserted through tissue intoa tumor. The end 14 is adapted to fit into a temperature controlmechanism 15, such as a resistance heater or a cryogenic element.

The temperature control mechanism 15 provides a controllable heating orcooling rate to the end 14 of the catheter 8 to maintain the needle-likeend 12 at a substantially constant temperature. For instance, thetemperature at the needle-like end 12 preferably ranges from 42.5° C.(108.5° F.) to 43.0° C. (109.4° F.), but this range may vary if anotherrange is deemed therapeutic for the diseased tissue. Controllableheating and cooling sources capable of maintaining temperatures in theappropriate range will operate satisfactorily, and these may include apreheated or pre-chilled liquid or a cryogenic fluid.

The catheter is an invasive device. The catheter 8 is inserted directlyinto a tumor or other diseased region of a body, and the catheter 8heats or cools the tumor to destroy it. A detachable handle (not shown)may be used for accurate placement, particularly for deep-seated tumorsor diseased areas. The handle may be removed and a clip-on temperaturecontrol mechanism 15 attached to the end 14 to control the temperatureof the needle-like end 12. The temperature of the needle-like end 12 ofthe catheter 8 is controlled to suit individual tumor requirements. Therate of heat delivered or removed is matched to the thermal conductivityof the tissue and the degree to which the tumor is perfused. The numberand depth of the catheters 8 to be inserted into a tumor or diseasedtissue depends on the volume and location of the diseased region withinthe body.

As shown in FIG. 2, an exterior portion of the stainless steel shaft 10of the catheter 8 is preferably insulated. This portion is preferablyinset so that the insulating material 22 can be deposited on the shaft10 without increasing the diameter of the shaft 10. The insulatingmaterial 22 minimizes the radial heat loss through the shaft 10 andminimizes damage to the normal tissue through which the catheter 8passes. FIG. 2A depicts the cross-section of the channel 17, and FIG. 2Bdepicts the cross-section of the channel 17 and the insulative layer 22.

FIG. 2 also illustrates the internal structure of a passivelycontrolled, hyperthermic, gas-loaded micro heat pipe that may be usedwithin the catheter 8. The heat pipe includes a channel 17 which has anon-condensible gas reservoir 20. The channel 17 is partially chargedwith an appropriate working fluid, such as pure water, methanol,ammonia, or nitrogen. The needle-like end 12 houses the condenser 16 ofthe heat pipe, and the end 14 houses the evaporator 18 of the heat pipe.In applications requiring the removal of thermal energy, such ashypothermia or the cooling of tissue, the roles of condenser andevaporator are reversed.

In most two-phase cycles, the presence of non-condensible gases createsa problem due to the partial blockage of the condensing area. Heat pipesare no exception. During normal operation, any non-condensible gasespresent are carried to the condenser and remain there, reducing theeffective condenser surface area. This characteristic, although normallyundesirable, can be used to control the direction and amount of heattransfer and/or the condenser temperature (i.e. the temperature at thetumor).

The heat pipe catheter 8 operates on the thermodynamic principal ofessentially constant temperature evaporation and condensation.Therefore, the temperature throughout the length of the catheter 8 issubstantially uniform. The temperature variation between the condenser16 and evaporator 18 regions in the heat pipe may be as little as ±0.1°C., depending upon the pressure, temperature, and working fluid used inthe heat pipe. When used for hyperthermia (heating the diseased tissue),the catheter 8 is coupled to a heat source that provides heat to thetumor, and when used for hypothermia (cooling the diseased tissue), thecatheter 8 is coupled to a cooling source that removes heat from thetumor.

In operation, the working fluid evaporates at the heat-source end 14 andcondenses at the tumor-heating section. FIGS. 2C and CI illustratealternative wicking configurations that carry the condensed workingfluid back to the evaporator.

FIGS. 3-8 illustrate a number of heat-transfer control techniques. Inthese figures, the vertical arrows depict the direction of heattransfer.

FIG. 3 illustrates an passively-controlled, gas-loaded heat pipemechanism 36 for the catheter 8. In this type of device, the thermalconductance of the heat pipe varies as a function of the "gas front"position. The term "gas front" refers to the vapor/noncondensible gasinterface. As the heat available at the evaporator varies, the vaportemperature varies and the noncondensible gas contained within thereservoir expands or contracts, moving the gas front. This in turnresults in a variation in the thermal conductance, i.e. as the heat fluxincreases, the gas front recedes and the thermal conductance increasesdue to the larger condenser surface area. In this way, the temperaturedrop across the evaporator and condenser can be maintained fairlyconstant even through the evaporator heat flux may fluctuate. This willprovide a constant temperature at the tumor site, preventing damage tosurrounding tissue.

While in most applications heat pipes operate in a passive manner,adjusting the heat flow rate to compensate for the temperaturedifference between the evaporator and condenser, several active controlschemes have been developed. Most notable among these are: (i)gas-loaded heat pipes with a feedback system, (ii) excess-liquid heatpipes, (iii) vapor flow-modulated heat pipes, and (iv) liquidflow-modulated heat pipes.

In the embodiment of FIG. 3, the gas volume at the reservoir end 38 ofthe heat pipe 36 is controlled passively by expansion and contraction ofthe non-condensible gas in the reservoir 38. Active control can .beachieved by heating or cooling the evaporator 18 as illustrated in FIG.4. The evaporator 18 is heated or cooled by coupling a temperaturecontrol mechanism 15 to a thin resistive coating 40 that is disposed,preferably by vapor deposition, on the shaft 10 of the catheter 8. Thecoating 40 is preferably Nichrome.

An actively controlled temperature control technique employs atemperature sensing device at the needle-like end 12 of the catheter 8to provide a feedback signal to the temperature control mechanism 15.FIG. 4 illustrates an actively-controlled, gas-loaded heat pipe 23 inwhich the gas volume at the reservoir end can be controlled externally.In this embodiment, a temperature-sensing device 25 at the condensorprovides a signal to the temperature control mechanism 15, which isillustrated in FIG. 4 as an evaporator heater 24, via a heatercontroller 27. The heater 24, when activated, can heat the gas containedin the evaporator, causing it to expand and thereby reducing thecondenser area.

FIG. 5 illustrates such the device 25 as a temperature-sensingthermocouple 26 for use with either hyperthermic or hypothermic microheat pipe catheters. FIGS. 5A and 5B illustrate the correspondingcross-sections. The temperature-sensing thermocouple 26 may befabricated through vapor deposition, chemical deposition, thermaldeposition, sputter deposition, plasma spraying or other similartechniques of depositing dissimilar metals on a portion of the shaft 10.The thermocouple mechanism 26 is created by first depositing anelectrically insulating layer 30, typically an oxide, onto the shaft 10and the needle-like end 12. A thin layer of electrically conductivematerial 32, such as copper, is deposited on the electrically insulatinglayer 30. A second thin layer of dissimilar electrically conductivematerial 34, such as iron, is then deposited over the first layer ofmaterial 32. Electrical connections, terminating in contact pads 28,from each of these two layers are fabricated by depositing thesematerials along the shaft 10 toward the end 14. The layers of material30, 32, and 34 are thin enough that they do not significantly increasethe diameter of the catheter 8.

Once the thermocouple 26 is formed, the difference in the electricalpotential at the junction of the materials 32 and 34 is correlative tothe temperature at the needle-like end 12 of the catheter 8. Thus, thistemperature signal can be delivered to the temperature control mechanism15 to more precisely control the temperature at the needle-like end 12of the catheter 8. The temperature sensing thermocouple 26 can be usedwith any of the passive temperature control mechanisms described withreference to FIGS. 5, 6, 7 and 8.

FIG. 6 illustrates a passively-controlled, vapor-flow modulated heatpipe mechanism 42 for the catheter 8 and which is particularly suitedfor hyperthermic treatments. Excess-liquid heat pipes operate in muchthe same manner as gas-loaded heat pipes but utilize excess workingfluid to block portions of the pipe and control the condenser size orprevent reversal of heat transfer. Vapor-flow-modulated heat pipesutilize a throttling valve to control the amount of vapor leaving theevaporator. In this embodiment, a throttling valve 41 controls theamount of vapor leaving the evaporator 18. An increase of thetemperature of the vapor in the evaporator 18 causes the baffle 46containing the control fluid to expand. This in turn closes down thethrottling valve 41, by pushing the plunger 48 into the opening 50 ofthe flange 52, and reduces the flow of vapor to the condenser 42. Thisembodiment is particularly useful in situations where the evaporatortemperature varies and a constant condenser temperature is desired.

FIG. 7 illustrates another passively-controlled, vapor-flow modulatedheat pipe 56 for use with the catheter 8 and which is particularlysuited for hypothermic treatments. A throttling valve 58 controls theamount of vapor leaving the evaporator 18. An increase of thetemperature of the vapor in the evaporator 18 causes the baffle 60containing the control fluid to expand. This causes the valve 58 to openby pushing the plunger 62 out of the opening 64 of the flange 66.

FIG. 8 illustrates a passively-controlled, liquid-flow modulated heatpipe 54 for the catheter 8. In this embodiment, two separate heatingstructures are utilized. A first wicking structure 68 transports liquidfrom the evaporator 18 to the condenser 16. A second wicking structure70 serves as a liquid trap. As the temperature gradient is reversed, theliquid moves into the trap 70 and starves the evaporator 18 of fluid,thus regulating the temperature of the condenser 16 to maintain thetumor at the predetermined temperature and to prevent damage to thesurrounding tissue.

The catheter 8 may be used to treat many varieties of cancer. Oneparticularly attractive feature of the catheter 8 is its ability totreat forms of cancer currently untreatable by invasive methods anddevices. The catheter 8 may be made thin enough to minimize hemorrhagingof the healthy tissue through which it passes, and its shaft 10 isinsulated to further reduce damage to healthy tissue. In each case, theneedle-sharp end 12 of the catheter 8 is inserted into the diseasedtissue 72. A heating or cooling source 15 is externally coupled to theend 14 to heat or cool the working fluid of the catheter 8.

FIG. 9 illustrates a method of treating diseased tissue 72 located nearthe spinal cord 76. A tumor near the spinal cord may not be amenable tosurgery due to the danger of severing nerves. However, the catheter 8may be inserted into a tumor near the spinal cord with little danger ofsevering nerves.

FIG. 10 illustrates a method of treating diseased tissue 72 located inthe prostrate gland 74, and FIG. 11 illustrates a method of treatingdiseased tissue 72 located in the breast 80. Although these tumors aregenerally treatable with surgical methods, the catheter 8 may be usedinstead to destroy these types of tumors without surgery.

FIG. 12 illustrates a method of treating diseased tissue 72 located nearthe eye 82. Like the spinal cord, surgery near the eye can also poseproblems, so the catheter 8 can be used to destroy the tumor and avoidsurgery. It should be noticed that the catheter 8 may be curved orflexible so that it can be inserted between the eyelid and the eye toreach the tumor 72 with the minimum of tissue penetration.

FIG. 13 illustrates a method of treating diseased tissue 72 located inor near the brain 84. Brain tumors must be located near the skull beforesurgery can be considered. Methods such as radiation therapy andchemotherapy are typically used to treat deeper tumors, and oftenwithout success. However, the catheter 8 may be inserted through braintissue with minimal damage due to the catheter's small diameter andinsulative covering. A hole is drilled in the skull and the catheter isinserted through the hole into the diseased tissue 72. Because thetemperature of the needle-like end 12 of the catheter 8 can becontrolled so precisely, the tumor can be therapeutically treated whilethe damage to surrounding tissue is minimized.

As is evident from the above descriptions, the catheter 8 delivers heatto or removes heat from a tumor or diseased region and maintains thetumor at a substantially constant temperature. The catheter 8 is asimple device that requires no complex external equipment, highvoltages, or wave energy transmissions. The catheter 8 may be eitheractively controlled through a self-contained unit and/or passivelycontrolled using one of several heat-pipe control mechanisms. Thecatheter 8 may be fabricated in different lengths and differentdiameters for specific tumor locations and volumes. For specificapplications, the catheter 8 may be curved or flexible to facilitate itsinsertion around an obstruction or to avoid the invasion of a particularorgan. Treatment of a cancerous tumor or diseased area may require anumber of catheters 8, depending upon the volume, location, andperfusion of the tumor. Finally, the catheter 8 may be designed tooperate at a preselected temperature, and the precise temperaturecontrol minimizes the amount of information required about the size,shape and density of the tumor being treated.

What is claimed is:
 1. A catheter comprising:a shaft having a first end,a second end, and an intermediate portion extending therebetween, saidfirst end having a needle-like shape for penetrating soft tissue andsaid second end being adapted to couple to a thermal transfer element; achannel being disposed within said shaft, said channel being chargeablewith a quantity of fluid, and said channel extending between said firstend and second end of said shaft and terminating in a fluid reservoir atsaid first end; a thermally insulative barrier disposed along saidintermediate portion of said shaft, said thermally insulative barrierprotecting tissue contacting said intermediate portion of said shaftfrom damaging temperature change; and a temperature sensing devicedisposed on the shaft, said temperature sensing device being adapted tocouple to said thermal transfer element to control the temperature ofsaid thermal transfer element.
 2. The catheter, as set forth in claim 1,wherein said temperature sensing device comprises a thermocouple.
 3. Thecatheter, as set forth in claim 2, wherein said thermocouple isdeposited by a process selected from the group consisting of vapordeposition, chemical deposition, thermal deposition, sputter deposition,and plasma spraying.
 4. The catheter, as set forth in claim 1, furthercomprising: a valve disposed within said channel for controlling fluidflow within said channel.
 5. The catheter, as set forth in claim 4,wherein said valve comprises:a flange being disposed in said channel,said flange separating said channel into an evaporator end and acondenser end, said flange having an aperture therein to facilitatevapor flow between said evaporator end and said condenser end; a bafflebeing disposed in said evaporator end of said channel and being filledwith a quantity of expandable fluid, said expandable fluid expanding tomove said baffle axially toward said flange and condensing to move saidbaffle axially away from said flange in response to temperaturevariations within said evaporator end; and a plunger being aligned withsaid aperture and being coupled to said baffle for axial movementtherewith to open and close said valve.
 6. The catheter, as set forth inclaim 1, wherein said channel comprises:an evaporator proximate saidfirst end of said shaft and a condenser proximate said second end ofshaft, said channel being chargeable with a working fluid, and saidchannel having a first wicking structure for transporting said workingfluid from said evaporator to said condenser and said channel having asecond wicking structure for preventing reverse flow of heat by trappingsaid working fluid.